Our overall objective is to understand the structure and operation of a model command system of intermediate complexity. The behavioral output, abdominal extension, is subserved in crustaceans by a limited number of command interneurons localized thus far in the rostral and caudal part of the CNS. One aim is to determine the interrelationships between these higher and lower level commands and the source of their inputs. A second aim is to record from selected command units while evoking behavior by natural stimulation. These data yield information on the sensory input to commands, on their recruitment as well as the information content of the signals they carry to the local circuits. Current evidence points to the importance of these local ganglionic circits in organizing reciprocity, in eliminating competing commands and reflexes and in selecting certain motor neurons which provide the appropriate motor program. The third aim is to examine the function of these local circuits with intracellular recording and current injection techniques. Using Lucifer as a stain and horseradish peroxidase as an EM marker, we plan to determine the morphology of these interneurons using light and electron microscopy.